An interior panel injection molded into a thin-plate shape using resin, for example, an instrument panel for a motor vehicle, is formed with openings for disposing a cluster bezel, a grove box, a ventilator grill, and the like. In addition, the concave-convex shape bent intricately is formed for mounting other various parts or devices to be mounted on the back side of the panel. As regards the front surface side of the interior panel, a design property not to expose the concave-convex which is meaningless as a design and does not have a good appearance is required.
The injection molding of the interior panel as described above is normally carried out using a injection molding die including a fixed cavity die having a supply channel for molten resin, and a movable core die opposing thereto. In general, the front surface side of the interior panel is formed by the cavity die and the back surface side thereof is formed by the core die. In the design of the injection molding dies as described above may result in inevitable generation of protruded-shaped portions which interfere in die cutting of the cavity die and the core die caused by the complex bent and concave-convex shape of the panel. The protruded shaped portions as such are referred to an “undercut portion”.
As a general countermeasure for molding the undercut portion, integration of a slide die which moves forward and backward along a desired direction, which is different from the die-cutting direction, into a part of the cavity die or the core die is contemplated.
However, the core die essentially requires the usage of a number of the slide dies. It is because molding of other various shaped portions such as pinning portions or screwing portions for connecting components on the back surface side of the panel are necessary in the core die in compensation for securing the design property on the front surface side of the panel. In other words, usage of the slide die for molding the undercut portions in the core die is remarkably limited essentially in design flexibility. Therefore, a technology which is able to solve the problem of molding of the undercut portions without using the slide die on the side of the core die is required.
As a countermeasure for the undercut portions which is conceivable in view of such point of view is to plug back undercut spaces on the side of the core die for the undercut portions of the panel with resin because it does not relate to the design property on the front surface side of the panel. Accordingly, the undercut portions on the side of the core die are eliminated. However, since the plugged-back portion as such is a resin bank, a molding sink as a depression of the resin surface occurs in association with cooling and solidification of the molten resin. When the molding sink occurs, there arises a problem of degradation of the design property on the front surface side of the panel. Since a superficial material at a portion corresponding to the portion of a substrate where the molding sink has occurred is depressed when the superficial material is adhered to the front surface side after having molded a panel body as the substrate, degradation of the design property is also resulted. When bonding an existing foam material layer and the superficial material with each other after having molded the substrate excluding the case of forming the foam material layer by expansion between the substrate and the superficial material, depression occurs in portions of the foam material layer and the superficial material corresponding to the portion of the substrate where the molding sink occurs, so that the degradation of the design property is resulted in the same manner as described above.
A gas injection method is disclosed in Cited Documents 1 and 2 shown below as a technology to prevent occurrence of the molding sink at the resin bank formed in part of the instrument panel or the like although it is different from the processing of the undercut portion. The gas injection method is a technology utilizing a point that the peripheral portion of the resin bank is cooled and solidified earlier, and the solidification of the center portion of the resin bank is delayed. In other words, unsolidified resin at the center portion of the resin bank is discharged by a gas pressure with precise timing when the peripheral portion of the resin bank is cooled and solidified. Accordingly, the resin bank having a hollow structure is made, so that occurrence of the molding sink is prevented as a result.    [Cited Document 1] JP-A-7-40372    [Cited Document 2] JP-UM-A-6-39555
However, when this method is applied to the processing on the undercut portion, it is necessary to mount a device such as a gas injector 37 shown in FIG. 3 in Cited Document 2 newly to, for example, the core die. In addition, it is necessary to form a space for receiving the molten resin discharged by the gas pressure additionally on the surface of the molding die. Therefore, the design flexibility of the injection molding die described above is further impaired.